Most terminated crystals form by 'growing' out of a rock. Which causes one side to be flat,(or whatever the shape of the rock it 'grew' out of was shaped like), and the other side to be 'terminated'. Meaning that the one side has a natural faceted end.

$\begingroup$My first guess would be initally forming on a relatively small condensation nucleus, which is then overgrown, but since I have no evidence at hand it might be completely wrong.. another way might be if the original crystal is broken off the rock, and then continues to grow at both ends.$\endgroup$
– hugovdbergApr 18 '14 at 14:49

$\begingroup$I strongly disagree with naught01. This question is specifically about Earth Science. I don't get why people try to push questions out of here that are interesting and relevant, just because they are a bit technical.$\endgroup$
– AntonioMay 6 '16 at 5:15

3 Answers
3

Double-terminated crystals can from by crystallizing from a melt. The crystallization nucleus has to float freely in the magma chamber. As long as no other crystals obstruct the growth the crystal will grow in its own characteristic form (euhedral). This happens for example with feldspar crystals. (Example: http://www.erdwissen.ch/wp-content/uploads/2010/12/granit-feldspat.jpg)

If the crystal forms in a fluid cavity it can form double-terminations when it doesn't grow with the c-Axis normal to the wall. Sometimes crystals also grow with their c-Axis parallel to the wall (Example: http://wannenkopfe.strahlen.org/titanite.html). Probably super cooled pegmatitic melts also form free floating double-terminated crystals like tourmaline.

On the smaller scale (microscope and electron microprobe) double terminated crystals are pretty common in metamorphic rocks. Sections cut parallel to the stretching lineation (or the lineation which elongated crystals aligned themselves to) will show many porphyroblastic crystals with double-terminations.

Double terminated crystals normally form in free floating pockets of liquid that slowly evaporated, leaving perfectly formed crystals with terminations on both sides.
Unlike usual quartz formations which grow in igneous rock, double terminated crystals are more than likely found in sedimentary rock laid down by subterranean marine deposits a long time ago. Occasionally, rare clusters of double terminated crystals form – very unique!

Why couldn't "evaporation" be a possibility here? We usually think of many igneous crystals forming during the cooling of magma, but evaporation too is a cooling process, we know. So if these molecules know how to arrange themselves from being in liquid form with say a wall of points growing out of a milky quartz matrix, for instance, why couldn't a much gentler and slower process of cooling happen with minerals suspended in water?

Back in the early 90s I spend a couple of days mining quartz points near Mt. Ida, Arkansas. I mostly worked on a single vein with two facing walls of large, optical-quality points growing toward one another. In between the two facing walls was packed about 6-8" that had filled in with a dense, rubbery red clay that had obviously settled into the vein over many many years and dried out. Suspended in that red clay at various points were, not double-terminated points, but tabular crystals that were not touching either wall of the vein, but had been apparently floating in their as it filled with clay-particle sediment. I have always wondered how they formed.

Thanks for the previous comments. I like the idea of a tiny nucleus begining the expansive growth into a double-termination, by contrast with the broad cooling of a wall of points, etc.

$\begingroup$Evaporation is not the correct term: it's something that happens on the surface. It is true, however, that boiling or phase separation may occur, enriching one phase and depleting the other in a certain chemical component (brine or vapour). This is commonly used to explain porphyry copper deposits, for example.$\endgroup$
– GimelistMay 4 '16 at 5:07

$\begingroup$This seems more like a question than an answer. If you have a new question you can ask one here. The answer space is reserved for answers only.$\endgroup$
– bonMay 4 '16 at 9:39